1. Field of the Invention
The invention relates to a semiconductor light-emitting element having a double hetero structure and, more particularly to a semiconductor light-emitting element such as a high output laser diode having an active layer formed of InGaAlP crystal and a light-emitting wavelength of about 600 nm.
2. Description of the Related Art
Semiconductor lasers of InGaAlP system have been widely used in reading light sources which are employed in a bar-code reader or a laser printer, etc. FIG. 1 is a cross sectional view showing an inner stripe (IS) structure as an example of a conventional laser of a InGaAlP system having a double hetero junction structure manufactured by a metalorganic chemical vapor deposition method (MOCVD) or a molecular beam epitaxial method (MBE).
In FIG. 1, as a first growth, an n-type GaAs buffer layer 22, an n-type In.sub.1-y (Ga.sub.1-s Al.sub.s).sub.y P cladding layer 23, an undopped In.sub.1-y (Ga.sub.1-x Al.sub.x).sub.y P active layer 24, a p-type In.sub.1-y (Ga.sub.1-s Al.sub.s).sub.y P cladding layer 25, and an n-type GaAs current stripe layer 26 are sequentially formed on an n-type GaAs substrate 21 by the MOCVD method. Then, a resist pattern having a stripe-shaped opening is formed on the upper portion by a photolithograph method. Thereafter, a stripe-shaped window portion 28, reaching p-type InGaAl P cladding layer 25, is formed by etching the n-type GaAs current stripe layer 26 with solution in which GaAs is etched and InGaAlP is not etched. Thereafter, the resist pattern is removed. Next, the second crystal growth is performed by the MOCVD method, thereby forming a p-type GaAs ohmic contact layer 27. In this way, a typical gain waveguide type semiconductor laser element can be obtained. In the IS laser element, Zn is normally used as the p-type dopant and Si is used as the n-type dopant.
The outline of the operation of the above IS laser will be explained as follow.
Metallic electrodes (not shown) are respectively adhered onto the upper and lower surfaces of the formed substrate, shown in FIG. 1, and a forward voltage (the electrode of the upper surface is positive and that of the lower surface is negative) is applied thereto when the laser is operated. A hole is injected into the active layer 24 from the p-type cladding layer 25 and an electron is injected into the active layer 24 from the n-type cladding layer 23. Since n-type current stripe layer 26 and p-type cladding layer 25 are reverse biased, an exciting current locally flows into the portion of p-type ohmic contact layer 27 embedding the stripe-shaped window portion 28. In other words, the implantation of the above carrier is limited to the portion which is close to the portion just below the stripe-shaped window portion 28.
A mixed crystal ratio of materials is determined in order to make the band-gap energy of the active layer 24 smaller than that of the cladding layers 23 and 25. Due to this, the implanted carrier is closed in the thin active layer 24 with a high density and the electron of the conduction band and the hole of the valence band are recombined, thereby emitting light having a wavelength corresponding to the band-gap energy.
On the other hand, the upper and lower surfaces of the active layer 24 are sandwiched between the cladding layers 23 and 25 whose index of refraction is smaller than that of the active layer 24. Due to this, the above-emitted light is closed in the active layer 24. In the case of the semiconductor laser, a active layer area close to the portion just below the stripe-shaped window portion 28, in its longitudinal direction, is a light-emitting region, and reflecting mirror surfaces (not shown) ar formed to be parallel with the both end surfaces of the light-emitting region. The active layer portion, which is sandwiched between the parallel reflecting mirror surfaces, becomes an optical resonator. Light emitted in the active layer is amplified by induced radiation, so that laser beam can be obtained.
Additionally, when layers 23 to 27 are formed on the substrate 21 one over another, n-type GaAs buffer layer 22 relaxes lattice mismatching. At the same time, n-type GaAs buffer layer 22 relaxes the increase of the lattice defect in the crystal growth layer or the generation thereof.
In the above-mentioned semiconductor light-emitting element, the following problems occur in the process of the crystal growth.
Specifically, dopant of the p-type cladding layer 25 hetero junctioning undoped active layer 24, such as Zn, disperses into undoped active layer 24 during the crystal growth. Due to this, there is a problem in that the conductivity type of active layer 24 becomes p-type, thereby increasing the band-gap energy in the active layer. For this reason, the oscillation wavelength is shifted to a short wavelength and the index of refraction of the active layer is decreased, so that the threshold current of the vibration is increased. For the above-mentioned reasons, the yield of the element and the reliance thereof are lowered.